AN-1114: Lowest Noise Zero-Drift Amplifier Has 5.6 nV/√Hz Voltage Noise Density

AN-1114
APPLICATION NOTE
One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com
Lowest Noise Zero-Drift Amplifier Has 5.6 nV/√Hz Voltage Noise Density
by Vicky Wong
INTRODUCTION
A sensor typically produces low output voltage and requires a
signal conditioning circuit with high gain and accurate dc performance. However, offset voltage, drift, and 1/f noise in amplifiers
cause errors, especially for dc or low frequency and low level
voltage measurements. Therefore, it is crucial to minimize offset
voltage and drift as well as eliminate 1/f noise for optimum
signal conditioning. Zero-drift amplifiers, designed to achieve
ultralow offset voltage and drift, high open-loop gain, high
power supply rejection, high common-mode rejection, and no
1/f noise, provide benefits to designers in precision applications.
AUTO-ZEROING vs. CHOPPING
A zero-drift amplifier, as the name suggests, has a near zero
offset voltage drift. The amplifier continuously self-corrects for
any dc errors, making it as accurate as possible. A zero-drift
amplifier can be designed with two different techniques: autozeroing or chopping. Each technique has its benefits and
drawbacks and is used in different applications.
Auto-zeroing uses the sample-and-hold technique and has more
in-band voltage noise due to noise foldback to the baseband.
Alternatively, chopping uses signal modulation and demodulation
and has lower baseband noise, but produces noise spectrums at
the chopping frequency and its harmonics. As a result, chopper
amplifiers are better suited for dc or low frequency applications
whereas auto-zero amplifiers are suitable for wider band
applications.
ADA4528-1 vs. TRADITIONAL CHOPPER
AMPLIFIERS
Traditionally, chopper amplifiers have fairly large baseband
noise (refer to Table 1) and low chopping frequencies, limiting
their usage to dc and sub-100 Hz applications. For applications
that require a chopper amplifier with a larger usable bandwidth,
Analog Devices, Inc., released the ADA4528-1, the lowest noise
chopper amplifier currently available in the semiconductor
industry. The ADA4528-1 employs a novel chopping technique
(with an autocorrection feedback loop) and has a chopping
frequency that is five to ten times higher than the chopping
frequencies of traditional chopper amplifiers.
With a chopping frequency at 200 kHz and an ultralow voltage
noise density of 5.6 nV/√Hz, this design breakthrough allows
the ADA4528-1 to be used in wider band applications where
traditional chopper amplifiers cannot be used in. In addition,
the ADA4528-1 also offers 0.3 µV offset voltage, 0.002 µV/°C
offset voltage drift, 158 dB common-mode rejection, and
150 dB power supply rejection. These specifications are ideal
for applications that require amplification of low level signals
in high gain and low noise precision applications. Such
applications include precision weigh scales, sensor front ends,
load cell and bridge transducers, interface for thermocouple
sensors, and medical instrumentation.
Rev. B | Page 1 of 8
AN-1114
Application Note
TABLE OF CONTENTS
Introduction ...................................................................................... 1
Auto-Zeroing vs. Chopping ............................................................ 1
Broadband Noise and External Source Resistance
Considerations ...............................................................................4
ADA4528-1 vs. Traditional Chopper Amplifiers ......................... 1
Calculation of Noise Contribution Referred To the Output ...5
Revision History ............................................................................... 2
Voltage Ripple ................................................................................6
ADA4528-1 Chopper Architecture ................................................ 3
ADA4528-1 as an Instrumentation Amplifier...............................7
Noise Characteristics........................................................................ 4
Conclusion..........................................................................................8
1/f Noise ......................................................................................... 4
REVISION HISTORY
4/14—Rev. 0 to Rev. B
Change to ADA4528-1 vs. Traditional Chopper Amplifiers
Section ................................................................................................ 1
5/11—Rev. 0 to Rev. A
Added ADA4528-1 vs. Traditional Chopper Amplifiers
Section ................................................................................................ 1
Changes to Figure 11 ........................................................................ 7
4/11—Revision 0: Initial Version
Rev. B | Page 2 of 8
Application Note
AN-1114
ADA4528-1 CHOPPER ARCHITECTURE
CHOP1
Gm3
CHOP2
Gm1
Gm2
OUT
IN–
NULL–
C3
NULL+
NF
C1
CHOP3
Gm4
ACFB
09798-001
Gm5
Figure 1. Amplifier Block Diagram
Figure 1 shows the ADA4528-1 amplifier block diagram. It
consists of a high dc gain path with autocorrection feedback
(ACFB) in parallel with a high frequency feedforward path.
The high dc gain path consists of an input chopping switch
network (CHOP1), a first transconductance amplifier (Gm1),
an output chopping switch network (CHOP2), and second and
third transconductance amplifiers (Gm2 and Gm3). The ACFB
loop contains a fourth transconductance amplifier (Gm4), a
chopping switch network (CHOP3), and a switched capacitor
notch filter (NF). Finally, the high frequency feedforward path
is made up of a fifth transconductance amplifier (Gm5). The
chopping frequency, fCHOP, of all chopping switch networks is
designed to operate at 200 kHz.
The high frequency feedforward path functions to amplify any
high frequency input signals near or above the chopping frequency. It also bypasses the phase shift introduced by the ACFB
loop. As a result, the ADA4528-1 has a standard −20 dB/decade
gain roll-off and a unity gain bandwidth of 4 MHz (see Figure 2).
This high bandwidth allows the ADA4528-1 to be configured in
high gain with sufficient loop gain for minimum gain error.
135
120
PHASE
90
90
60
45
GAIN
30
0
–30
1k
The input baseband signal is initially modulated by CHOP1.
Next, CHOP2 demodulates the input signal and modulates the
initial offset and 1/f noise of Gm1 to the chopping frequency.
Gm4 in the ACFB loop then senses the modulated ripples at the
Rev. B | Page 3 of 8
0
VSY = 5V
RL = 10kΩ
CL = 100pF
GBP = 4MHz
ΦM = 57 Degrees
10k
PHASE (Degrees)
IN+
In addition, CHOP3 modulates the baseband desired signal at
the output of CHOP2 to the chopping frequency. The notch
filter, synchronized with the clock frequency, filters out signals
at the chopping frequency, and thus the modulated components.
Therefore, the ACFB loop selectively suppresses unwanted
offset voltage and 1/f noise without perturbing the desired input
baseband signal.
–45
100k
1M
–90
10M
FREQUENCY (Hz)
Figure 2. Open-Loop Gain and Phase vs. Frequency
09798-002
C2
output of CHOP2. The ripple is demodulated to the dc domain
by CHOP3, passed through the notch filter, and fed into the
null input terminals of Gm1 (NULL+ and NULL−). Gm1
proceeds to null out the initial offset and 1/f noise, which
otherwise appears as modulated ripples at the overall output. This
way, the continuous ACFB loop suppresses the modulated ripple.
OPEN-LOOP GAIN (dB)
The ADA4528-1 features a novel, patented technique that
suppresses offset-related ripple in a chopper amplifier. Unlike
other chopper techniques that filter the ripple in the ac domain,
this technique nulls the amplifier’s initial offset in the dc
domain. The ADA4528-1 nulls out the offset with a local
feedback loop, referred to as autocorrection feedback (ACFB),
thus preventing ripple at the overall output.
AN-1114
Application Note
NOISE CHARACTERISTICS
100
Zero-drift amplifiers, however, do not exhibit 1/f noise. They
reshape the voltage noise to eliminate 1/f noise. Because 1/f noise
appears as a slow varying offset, it can be effectively eliminated
by the chopping technique. The correction becomes more
effective as the noise frequency approaches dc, eliminating the
tendency of the noise to increase exponentially as frequency
decreases. Figure 4 shows the voltage noise density of the
ADA4528-1 with no 1/f voltage noise. The chopping technique
results in the ADA4528-1 having only 97 nV p-p of voltage
noise from 0.1 Hz to 10 Hz at a supply voltage of 2.5 V, much
lower noise at low frequency than standard low noise amplifiers
that are susceptible to 1/f noise.
VSY = 5V
VCM = VSY/2
100
BROADBAND
NOISE
1/f NOISE
1
10
NO 1/f NOISE
10
1
1
10
100
1k
10k
FREQUENCY (Hz)
Figure 4. ADA4528-1 Zero-Drift Amplifier: Voltage Noise Density vs.
Frequency
BROADBAND NOISE AND EXTERNAL SOURCE
RESISTANCE CONSIDERATIONS
The ADA4528-1 is the lowest noise zero-drift amplifier currently
available in the industry with 5.6 nV/√Hz of voltage noise
density at 1 kHz (at VSY = 2.5 V, AV = 100). It is, therefore,
important to consider the external input source resistance to
maintain total low noise performance of the system.
The total input referred noise (en total) that must be considered
in any amplifier design is primarily a function of three types of
noise: input voltage noise, input current noise, and thermal
(Johnson) noise from the external resistors. The input voltage
noise and input current noise are usually stated in the electrical
specifications section of a data sheet. The thermal noise of an
external source resistor can be calculated from the following
equation:
VRS = √4 kTRS
10
100
1k
FREQUENCY (Hz)
10k
where:
k is the Boltzmann constant (1.38 × 10−23 J/K).
T is the temperature in Kelvin (K).
RS is the total input source resistance (Ω).
09798-003
VOLTAGE NOISE DENSITY (nV/√Hz)
1000
VOLTAGE NOISE DENSITY (nV/√Hz)
1/f noise, also known as pink noise or flicker noise, is inherent in
semiconductor devices and increases as frequency decreases.
Therefore, it dominates noise at dc or low frequency. The 1/f
corner of an amplifier is the frequency at which the flicker noise
is equal to the broadband noise. Figure 3 shows an example of
an amplifier that does not have the zero-drift technology; the
1/f corner frequency is at 800 Hz. For dc or low frequency
applications, 1/f noise is a major noise contributor and can
cause a significant output voltage offset when amplified by the
circuit noise gain.
VSY = 5V
AV = 100
VCM = VSY/2
09798-004
1/F NOISE
Figure 3. Non zero-Drift Amplifier: Voltage Noise Density vs. Frequency
The three uncorrelated noise sources can be summed up in a
root sum squared (rss) manner in the following equation:
en total = √[en2 + 4 kTRS + (in × RS)2]
where:
en is the input voltage noise of the amplifier (V/√Hz).
in is the input current noise of the amplifier (A/√Hz).
The total equivalent rms noise over a specific bandwidth is
expressed as
en,RMS = en total √BW
where BW is the bandwidth in hertz.
Rev. B | Page 4 of 8
Application Note
AN-1114
This analysis is valid for a flatband noise calculation. If the
bandwidth of concern includes the chopping frequency, more
complicated calculations must be made to include the effect of
the noise spectrum at the chopping frequency (see Figure 8).
Voltage noise density is sometimes dependent on the amplifier
gain configuration. Figure 5 shows the voltage noise density vs.
closed-loop gain of a zero-drift amplifier of a leading competitor. The voltage noise density of the amplifier increases from
11 nV/√Hz to 21 nV/√Hz as closed-loop gain decreases from
1000 to 1. Figure 6 shows the voltage noise density vs. frequency
of the ADA4528-1 for three different gain configurations, AV = 1,
10, and 100. The ADA4528-1 offers a constant voltage noise density of 6 nV/√Hz to 7 nV/√Hz regardless of gain configurations.
CALCULATION OF NOISE CONTRIBUTION
REFERRED TO THE OUTPUT
Figure 7 shows the ADA4528-1 in a noninverting configuration.
A calculation of noise contribution from the external resistors,
amplifier voltage, and current noise referred to the output
(RTO) is as follows:
Noise gain = 1 + RF/RS
VRS = √4 kTRS
VRF = √4 kTRF
Error due to RS thermal noise = VRS × RF/RS
Error due to RF thermal noise = VRF
Error due to amplifier voltage noise = en × (1 + RF/RS)
VSY = 5V
f = 100Hz
COMPETITOR A
20
Error due to amplifier current noise = in × RF
Refer to Table 1 for calculation results.
RF
10kΩ
16
VIN
8
VOUT
ADA4528-1
09798-008
RS
100Ω
12
Figure 7. Noninverting Gain Configuration
4
0
1
10
100
1000
CLOSED-LOOP GAIN (V/V)
09798-006
VOLTAGE NOISE DENSITY (nV/√Hz)
24
Figure 5. Competitor A: Voltage Noise Density vs. Closed-Loop Gain
VSY = 5V
VCM = VSY/2
10
AV = 1
AV = 10
AV = 100
1
1
10
100
1k
10k
FREQUENCY (Hz)
09798-007
VOLTAGE NOISE DENSITY (nV/√Hz)
100
Figure 6. ADA4528-1: Voltage Noise Density vs. Frequency
Table 1. Calculated Output Noise (VSY = 5 V)
Noise Source
RS
RF
Voltage Noise
Current Noise
1
Value (at f = 1 kHz)
100 Ω
10 kΩ
5.9 nV/√Hz
0.5 pA/√Hz
Thermal Noise (nV/√Hz)
1.283
12.83
N/A 1
N/A1
Total Noise RTO (nV/√Hz)
128.3
12.83
595.9
5
N/A means not applicable.
Rev. B | Page 5 of 8
Output Noise Contribution (%)
4.43
0.04
95.52
0.01
AN-1114
Application Note
VOLTAGE RIPPLE
CF
220nF
VOUT
First, the voltage ripple is part of the residual ripple associated
with the initial offset Gm1 (see Figure 1). This ripple creates
higher noise spectrums at the chopping frequency (200 kHz)
and its harmonics. Figure 8 shows the voltage noise density of
the ADA4528-1 vs. frequency in three different gain configurations. The amplifier in the unity gain configuration has a noise
spectrum of 50 nV/√Hz at 200 kHz. This noise spectrum is
significant when the op amp has a closed-loop bandwidth that is
greater than the chopping frequency. However, with a higher
gain, the noise spectrum becomes less significant due to the
natural gain roll-off characteristic of the amplifier. Therefore, the
ADA4528-1 is excellent for use in dc high gain configuration
with its ultralow noise, offset voltage, and drift capability.
Figure 9. Reducing Noise Using a Feedback Capacitor
100
1
WITH CF
0.1
10
100
1k
10k
100k
1M
FREQUENCY (Hz)
Figure 10. Voltage Noise Density with Feedback Capacitor
10
1
In addition, note that all zero-drift amplifiers are susceptible to
residual ripple of the initial offset and intermodulation distortion.
AV = 1
AV = 10
AV = 100
100
1k
10k
FREQUENCY (Hz)
100k
1M
09798-009
VOLTAGE NOISE DENSITY (nV/√Hz)
VSY = 5V
VCM = VSY/2
10
WITHOUT CF
The second source of voltage ripple is the result of intermodulation between the chopping frequency (fCHOP) and the input signal
frequency (fIN). Intermodulation distortion (IMD) is a function
of the input signal frequency, and larger errors are introduced as
the input signal frequency approaches the chopping frequency.
This intermodulation creates a noise spectrum at the secondorder IMD products at fCHOP ± fIN, third-order IMD products
at 2fIN ± fCHOP and 2fCHOP ± fIN, and so on. The ADA4528-1 produces very low intermodulation distortion in comparison to other
zero-drift amplifiers. An input signal of 500 mV p-p voltage at
180 kHz produces 14.6 µV rms of distortion at 20 kHz.
100
1
10
09798-013
VOLTAGE NOISE DENSITY (nV/√Hz)
VSY = ±2.5V
To suppress noise at the output, place a feedback capacitor
around the amplifier. Figure 9 and Figure 10 show the configuration and the corresponding voltage noise density vs. frequency
graph. The feedback capacitor reduces the amplifier bandwidth
in an effort to reduce noise.
0.1
RF
2kΩ
RS
200Ω
09798-010
Although chopper amplifiers null out initial offset voltage,
voltage ripples still exist. Two sources contribute toward these
voltage ripples.
Figure 8. Voltage Noise Density vs. Frequency at Different Closed-Loop Gains
Rev. B | Page 6 of 8
Application Note
AN-1114
ADA4528-1 AS AN INSTRUMENTATION AMPLIFIER
The extremely low offset voltage and drift, high open-loop gain,
high common-mode rejection, and high power supply rejection
of the ADA4528-1 make it an excellent op amp choice as a
discrete, single-supply instrumentation amplifier.
Figure 11 shows the classic 3-op amp instrumentation amplifier
using the ADA4528-1. The key to high CMRR for the instrumenttation amplifier are resistors that are well matched for both the
resistive ratio and relative drift. For true difference amplification, matching of the resistor ratio is very important, where
R5/R2 = R6/R4. The resistors are important in determining
the performance over manufacturing tolerances, time, and
temperature. Assuming a perfect unity gain difference amplifier
with infinite common-mode rejection, a 1% tolerance resistor
matching results in only 34 dB of common-mode rejection.
Therefore, at least 0.01% or better resistors are recommended.
VIN1
A1
RG1
RG2
R5
R1
R2
R3
R4
A3
A2
VOUT
R6
RG1 = RG2, R1 = R3, R2 = R4, R5 = R6
VOUT = (VIN2 – VIN1) (1 + R1/RG1) (R5/R2)
09798-012
VIN2
Figure 11. Discrete 3-Op Amp Instrumentation Amplifier
To build a discrete instrumentation amplifier with external
resistors without compromising on noise, pay close attention to
the resistor values chosen. RG1 and RG2 each has thermal noise
that is amplified by the total noise gain of the instrumentation
amplifier and, therefore, should be chosen sufficiently low to
reduce thermal noise contribution at the output and yet provide
an accurate measurement. Table 2 shows the external resistors
noise contribution referred to the output (RTO).
Table 2. Thermal Noise Contribution Example
Resistor
RG1
RG2
R1
R2
R3
R4
R5
R6
Value
(Ω)
400
400
10 k
10 k
10 k
10 k
20 k
20 k
Resistor Thermal
Noise (nV/√Hz)
2.57
2.57
12.83
12.83
12.83
12.83
18.14
18.14
Thermal Noise
RTO (nV/√Hz)
128.30
128.30
25.66
25.66
25.66
25.66
18.14
18.14
Note that A1 and A2 have a high gain of 1 + R1/RG1. In this
case, the input offset voltage and the input voltage noise of the
amplifiers are important. Similar to RG1 and RG2, the input offset
voltage and the input voltage noise of the amplifiers are amplified by the overall noise gain. Therefore, use a high precision,
low offset voltage and low noise amplifier for A1 and A2, such
as the ADA4528-1. On the other hand, A3 operates at a much
lower gain and has a different set of op amp requirements. Its
input noise, referred to the overall instrumentation amplifier
input, is divided by the first stage gain and is not as important.
For dc and low frequency application that requires minimal
voltage drift, use zero-drift amplifiers, such as the AD8538
or AD8628 for A3. If voltage drift is not a concern, use
the AD8603.
Rev. B | Page 7 of 8
AN-1114
Application Note
CONCLUSION
For a low noise op amp selection table, refer to the AN-940
Application Note, Low Noise Amplifier Selection Guide for
Optimal Noise Performance.
In summary, the main features of the ADA4528-1 are
•
•
•
•
•
•
Ultralow offset voltage and drift
No 1/f voltage noise
Ultralow voltage noise density
High common-mode rejection
High power supply rejection
Rail-to-rail input and output
For more information on noise, watch the three-part webinar
series, Noise Optimization in Sensor Signal Conditioning
Circuits.
The design architecture specifically targets high gain precision
signal conditioning applications that require accurate and stable
performance in the dc or low frequency bandwidth.
•
•
•
Part 1
Part 2
Part 3
For a selection of other zero-drift amplifiers, see Table 3.
Table 3. Single Zero-Drift Amplifiers
Part
Number
ADA4528-1
AD8628
AD8638
AD8538
ADA4051-1
VSY (V)
2.2 to 5.5
2.7 to 5
5 to 16
2.7 to 5.5
1.8 to 5.5
VOS Max
(µV)
2.5
5
9
13
15
TCVOS Max
(µV/C)
0.015
0.02
0.06
0.1
0.1
GBP
(MHz)
4
2
1.35
0.43
0.115
ISY/Amp
Max (mA)
1.7
1
1.3
0.18
0.018
eN at 1 kHz
(nV/√Hz)
5.6
22
60
50
95
©2011–2014 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
AN09798-0-4/14(B)
Rev. B | Page 8 of 8
CMRR
Min (db)
135
110
118
110
105
PSRR
Min (db)
130
115
127
105
110
IB Max
(pA)
400
100
40
25
50
R-R
In
Yes
Yes
No
Yes
Yes
R-R
Out
Yes
Yes
Yes
Yes
Yes
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